1. Field of the Invention
The present invention relates to a seat back of vehicle seat with a movable headrest. In particular, the invention is directed to a structure of seat back of seat for vehicle which permits a headrest mounted in the seat back to be displaceable in forward and upward directions in response to a rear-end collision occurred to the vehicle.
2. Description of Prior Art
Among vehicle seats with headrest, there has been known a vehicle seat of such type that, responsive to a rear end collision, a headrest mounted thereon is subjected to forward movement under a backward force of inertia applied from a waist portion of the occupant and moved toward a head portion of occupant on the seat. For example, the Japanese Laid-Open Patent Publication No. 2001-26232 discloses the vehicle seat of this kind. According thereto, the seat back thereof includes a seat back frame and an auxiliary frame, such that a lower portion of the auxiliary frame is rotatably connected via a lower link with the seat back frame. A headrest is fixed on the top of the auxiliary frame to support a head of seat occupant, while a pressure receiving portion is provided in the lower region of the auxiliary frame at a point where a waist portion of the seat occupant is positioned. When a rear-end collision occurs, a backward load is applied to that pressure receiving portion from the seat occupant's waist portion which is moved backwardly under an inertia caused by the rear-end collision. Then, due to such backward load, the auxiliary frame is rotated forwardly of the seat back frame relative to the lower link, which causes simultaneous forward displacement of both headrest and upper portion of the auxiliary frame, thereby providing a quick support to the occupant's head and upper body portion.
In this conventional seat back structure, the auxiliary frame is normally biased by a biasing means, such as a spiral spring, in a direction to a given home position against the forward movement stated above. However, such biasing arrangement gives a full biasing force of the biasing means directly to both of the auxiliary frame and headrest, which undesirably results in giving a counterforce or resistance to the afore-discussed quick forward displacement of those auxiliary frame and headrest in the case of rear-end collision. Consequently, this prior art has been with the following problems: (i) When a minor rear-end collision occurs under a relatively slow speed of rearward vehicle, the auxiliary frame is slowly displaced in the forward direction due to a weak inertia or a week backward load being applied from the seat occupant and therefore, such week backward load is almost counterbalanced by the biasing force of the biasing means, so that the forward displacement of auxiliary frame becomes slow and therefore the occupant's head is not quickly received and supported by the headrest, and (ii), on the other hand, when a great abrupt rear-end collision occurs under a high speed of rearward vehicle, a great load is naturally applied from the seat occupant's waist portion to the pressure receiving portion, but a much greater load than such load is applied from the seat occupant's head to the headrest, as a result of which, the headrest, which is resiliently biased by the biasing means to a home position, is brought to keen contact with the seat back frame, overcoming the biasing force, thereby giving a corresponding greater load to the seat back frame. In the latter case, an excessive great load is directly exerted on the seat back frame from the headrest, which in turn causes an abrupt excessive backward inclination of the seat back frame or seat back. Hence, it is highly possible that a serious damage will be given to both seat back and seat occupant. Naturally, this undesired condition requires a rigid and robust construction of the seat back frame per se, which includes increasing a thickness of the seat back frame enough to withstand the foregoing great load applied from the headrest. But, such reinforcement of seat back frame will undesirably increases the weight of seat as well as costs involved.